The present invention relates to a coil spring formed by coiling a spring wire having a non-circular cross-section into a helical shape.
A commonly used coil spring 01 (FIG. 1) is a spring formed by coiling a spring wire 02 having a circular cross-section into a spiral shape. When an axial tension load P.sub.1 is exerted upon this coil spring 01, a torsion shearing stress in the direction shown by arrows A.sub.1 is generated in the spring wire 02, and when an axial compression load P.sub.2 is exerted, a torsion shearing stress in the direction shown by arrows A.sub.2 is generated. Due to the fact that the spring wire 02 is curved, this torsion shearing stress becomes maximum at the inner circumference of the coil. In this connection, the maximum shearing stress (.tau..sub.max) composed of this maximum shearing stress generated by torsion and the direct shearing stress generated by the axial load P, is represented by the following formula: EQU .tau..sub.max =8DP/nd.sup.3 {(4C-1)/(4C-4)+0.615/C}
where D represents an average diameter of the coil, d represents a diameter of the spring wire, and C (=D/d) represents a spring index.
Due to the fact that this maximum shearing stress (.pi..sub.max) is generated at the inner circumference of the coil, when an excessive repeated load is exerted upon the coil spring 01, there is a tendency that cracks may be generated in the spring wire 02 at the inner circumference of the coil. In order to obviate this shortcoming, there has been proposed a coil spring 03 (FIG. 2) formed by modifying a cross-section of a spring wire 04 into an oval shape and coiling the spring wire 04 with a major diameter 05 of the oval directed in the direction transverse to a center line L of the spring (Japanese Utility Model publication No. 27-3261 (1952)). In this coil spring 03, shearing stresses at the inner and outer circumferences, respectively, of the coil under action of an axial load are small at the inner circumference and large at the outer circumference as compared to those in a coil spring formed of a spring wire having a circular cross-section with a diameter equal to a minor diameter 06 of the spring wire 04, and moreover, owing to reduction of the maximum shearing stress (.tau..sub.max) at the inner circumference of the coil, a difference between the shearing stresses at the inner and outer circumferences becomes small. Therefore, an energy efficiency of a spring can be improved, and a scope of use is enlarged.
However, in the spring wire 04 of the coil spring 03, due to the fact that a radius of curvature at the inner circumferential portion of the coil is small, stress distribution on the peripheral surface of the spring wire is uneven, and upon the coiling work it was considered difficult to coil the spring wire in such manner that the extension line of the major diameter 05 of the spring wire 04 may intersect with the center line L at right angles. In order to obviate this shortcoming, there has been proposed a coil spring 03A, in which the side having a small radius of curvature of the spring wire 04A of oval shape in cross-section is directed towards the outer circumference of the coil (Laid-Open Japanese Patent Specification No. 60-69337 (1985)).
Even in the case of employing such spring wire 04A of oval shape in cross-section, if the coiling work is carried out simply in the conventional manner, torsion would be naturally generated in the spring wire 04A, and the extension line of the major diameter 05A would not become perpendicular to the center line L. However, here it is assumed that the coil spring 03A shown in FIG. 4 in which the extension line of the major diameter 05A intersects with the center line L at right angles, has been produced. In this case, if an axial load in the direction of compression is applied to the coil spring 03A, then torsional forces in the direction shown by arrows B are exerted upon the spring wire 04A in this figure, reference character S designates a reference plane of extension and contraction, and the spring wire 04A would be twisted depending upon the magnitude of the applied forces, resulting in increase of the maximum shearing stress (.tau..sub.max) under the loaded condition. Whereas, if the axial load is applied in the tensional direction, the torsion of the spring wire 04A would be generated in the opposite direction (in the direction shown by arrows C).
In practice, if coiling is carried out in a simple manner as described above, the coil springs 03 and 03A both have a tendency that torsion in the direction shown by arrows B would be naturally generated. FIG. 5 is a diagram showing influences of the naturally generated torsion upon the maximum shearing stress with respect to the coil spring 03, the abscissa representing a torsion angle (.alpha.) of the spring wire, and the ordinate represents a proportion of increase of the maximum shearing stress (%). In the upper portion of FIG. 5 is shown by dash lines the condition where the spring wire 04 of the coil spring 03 has been twisted by an angle (.alpha.) from a target coiled attitude (an attitude having the extension of the major diameter 05 intersected at right angles with the center line L as shown by solid lines), that is, the condition where torsion has been naturally generated as a result of the conventional coiling. As will be seen from this figure, if the torsion angle (.alpha.) becomes large, the maximum shearing stress (.tau..sub.max) generated at the inner circumference of the coil spring 03 would be increased.
The direction of this naturally generated torsion is an unfavorable direction for a compression coil, but is a favorable direction for a tension coil. More particularly, in a compression coil, when a compression load that is a load in use is exerted thereupon, a spring wire would be twisted up to a far larger angle than the above-mentioned naturally generated torsion, but in a tension coil, when a tension load that is a load in use is exerted thereupon, the above-mentioned naturally generated torsion would be restored.
Here, what is to be kept in mind is that in the case where a pitch of a coil spring is not uniform over its entire length but the coil spring consists of a larger pitch portion and a smaller pitch portion, the torsion generated by application of a load is different between the large pitch portion and the small pitch portion, and a torsion angle of the large pitch portion is smaller than a torsion angle of the small pitch portion.